1. Field of the Invention
The present invention relates generally to a radar apparatus and, more particularly, to an Automatic Radar Plotting Aid (ARPA) configured based on a radar apparatus which transmits and receives signals having different pulselengths assigned individually to plural range scales in use through a single antenna.
2. Description of the Related Art
A radar apparatus conventionally used on a ship, for instance, transmits a specific pulse signal and receives an echo signal reflected by surrounding targets through an antenna. The radar apparatus determines angular direction, or bearing (θ), of a particular target from the direction of the antenna as well as a range (r) to the target based on the time elapsed from transmission of the pulse signal to reception of the echo signal.
The pulse signal transmitted from the antenna has a pulselength which is determined in accordance with a detection range and range discrimination that are required. Typically, the pulselength is determined for each range scale selected for on-screen presentation. For example, a short pulselength is selected to achieve higher range discrimination on a short range scale as echoes received from nearby targets are generally strong, whereas a long pulselength is selected to receive a larger amount of information and thereby achieve improved target detectability on a long range scale as echoes from far targets weaken.
If the transmission pulselength is lengthened to improve long-range target detectability, however, the range discrimination deteriorates. Thus, there arises a problem that nearby targets can not be discriminated from one another when these targets are observed on a long range scale. The conventional radar apparatus exhibits this drawback most conspicuously particularly when viewing nearby targets on a long range scale. For this reason, it has been necessary for a radar observer to switch the radar apparatus to an optimum range scale depending on the distance between own ship and targets of interest.
Although some radar apparatuses offer a “zoom” function which is conventional, the zoom function simply displays an enlarged picture obtained by “zooming in” part of the echo signal received on the range scale in use and, thus, the enlarged part of the echo signal is not displayed with increased range discrimination. Therefore, the zoom function does not provide any solution to the aforementioned drawback of the conventional radar apparatus.
Japanese Patent Application Publication No. 1992-98179 and Japanese Patent No. 3507717 each disclose a technique which enables a radar apparatus to transmit signals having different pulselengths assigned individually to two or more range scales according to a selected transmit pulse sequence and simultaneously display radar pictures on two or more range scales on a display unit. This technique, if implemented in a radar apparatus, makes it possible to simultaneously present echo signals obtained with multiple pulselengths on different range scales with high target detectability.
Today, radars provided with an ARPA function are widely used onboard ships to aid in preventing a collision with a target ship or other objects.
An ARPA acquires user-selected and/or automatically selected targets, automatically tracks the acquired targets and calculates such motion-related information as course and speed of each tracked target by mathematically processing target data obtained by digitizing radar echo signals detected from a specific detection area. The ARPA presents the motion-related information on the targets in an easy-to-understand manner and, when a dangerous situation occurs, generates a signal to warn an observer of that situation.
The aforementioned technique of Japanese Patent Application Publication No. 1992-98179 and Japanese Patent No. 3507717, if implemented in an ARPA, poses a problem that the result of acquisition and tracking of the same target would differ depending on which one of the multiple pulselengths is used. This is because echoes of two nearby targets which are separated from each other on a short pulselength can merge into a single target echo when the ARPA is switched to a long pulselength which results in a poor range discrimination.
Especially when the ARPA is operated in a mode in which multiple radar pictures obtained with different pulselengths are simultaneously displayed on a single screen, the ARPA function will provide different results for the targets detected with the short pulselength and the targets detected with the long pulselength, thereby causing confusion to the user.